Miniaturized trimmer potentiometer



km 6?, 3% e. W. YUNGBLUT ET.AL .3,'488,6'18 I MINIATURIZED TRIMMER POTENTIOMETER Filed. July 1, 1968 I 2 Sheets-Sheet 1 INVENTORS CHARLES W. YUNGBLUT VICTOR G. MATH/501v RALPH L, COOK 3m H7 c W, YUNGBLUT ET AL 3,488,618

MINIATURIZED TRIMMER POTENTIOMETER Filed July 1. 1968 2 Sheets-Sheet 2 M 1 11' INVENTORS $9 {09 CHARLES W. YUNGBLUT. VICTOR G MATHISON 24 k 22 RALPH L. COOK M 2 .'#9%,MM,4mg4M United States Patent US. Cl. 338-164 15 Claims ABSTRACT OF THE DISCLOSURE A miniaturized trimmer potentiometer of the type having an open-sided housing mounting a worm gear and wormwheel and enclosed by a wafer of substrate having an annular resistance element and with a spring wiper interposed between the wormwheel and resistance element for adjusting the amount of resistance as the worm gear is rotated. The wiper is formed of flat spring stock and has a pair of integral outwardly extending arms curled inwardly in opposition to one another to frictionally embrace a stub shaft on the wormwheel. Also included in the wiper is a third arm for resiliently engaging a central contact as well as a resilient member for engaging the resistance element.

Great strides have been made in the miniaturization of electronic circuits, first with the advent of the transistor and subsequently with the integrated circuit. Not only have circuit assemblies become more compact by several orders of magnitude but reliability has been greatly improved, particularly in the case of devices intended for military or space usage. This has placed potentiometer designers and manufacturers under a great deal of pressure to bring about a corresponding decrease in size and increase in the reliability in their product. However, efforts directed toward the miniaturization of potentiometers having a maximum dimension on the order of 4 inch have not been wholly successful since manufacturers have, for the most part, attempted simply to scale down the size of larger units without substantially changing the design and without corresponding reduction in the number of parts.

It is, accordingly, an object of the present invention to provide a trimmer potentiometer measuring on the order of A inch overall which has an optimum combination of features demanded by military and space specifications, i.e., which is almost perfectly reliable, which cannot be mechanically damaged due to misuse, Which is capable of large overload without catastrophic failure, which is easily and quickly adjustable, retaining its adjustment in spite of extreme shock or vibration, and which is almost universal in application, capable of three different types of lead mounting employing the same internal mechanism and the same basic housing structure. It is, moreover, an object to provide a sub-miniature trimmer potentiometer which is humidity proof and capable of operating stably over an extremely wide range of ambient temperature.

-It is, moreover, an object of the present invention to provide a miniaturized trimmer potentiometer which is, in spite of its unusual capabilites, formed of parts which are inherently inexpensive, and which may be easily and quickly assembled and sealed with a final epoxy potting step.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

FIGURE 1 is a perspective view of a trimmer potentiometer constructed in accordance with the present in- "ice vention and of the Type X mounting configuration with the leads projecting laterally with respect to the adjusting screw;

FIG. 2 is an exploded view of the device shown in FIG. 1;

FIG. 3 is a vertical section taken along the line 33 in FIG. 4;

FIG. 4 is a bottom view in partial section looking along the line 44 in FIG. 3;

FIG. 5 is a bottom view of the housing with the screw in partial section;

FIG. 5a is a diagram showing forceable entry of the adjusting screw;

FIG. 6 is a plan view of the spring wiper element with the profile of the blank from which it is made indicated by the dot-dash outline;

FIG. 7 is a view of the completed wiper element looking along the line 7-7 in FIG. 6; and

FIG. 8 is a further view of the wiper looking along the line 88 in FIG. 7.

While the invention has been described in connection with a preferred embodiment, it will be understood that We do not intend to be limited to the particular embodiment shown but on the contrary intend to cover the various alternative and equivalent constructions included Within the spirit and scope of the appended claims.

Turning now to the drawings, there is disclosed in FIGURE 1, a trimmer potentiometer 10 constructed in accordance with the present invention having terminal leads 11, 12, 13 projecting from a housing 14. An adjusting screw 15, having a slotted head 16, serves to adjust the setting of the internal wiper. The lead configuration, in which the leads extend at right angles to the adjusting screw, is referred to in the art as Type X. It is one of the features of the present construction that it may be readily adapted to Type W configuration, in which the leads extend parallel to the adjusting screw or, indeed, to the Type P in which the leads extend along the third possible axis. While the trimmer potentiometer has been greatly enlarged in the drawings, it will be understood that the maximum dimension of the housing is preferably on the order of inch.

Turning next to the internal construction shown in FIGS. 2, 3 and 4, the housing 14 is in the form of a rectangular box molded of thermoplastic material having a first side 21 penetrated by the adjusting screw 15, an opposite side 22, remaining sides 23, 24, and an end wall 25. An internal shoulder 26 which extends about the inside periphery of the housing defines a central receptacle 27. An annular e-mbossment 28, is centered in the receptacle.

In accordance with the present invention a wormwheel is provided in the receptacle mounted on the embossment for rotation by the adjusting screw. A wafer of substrate, seated on the internal shoulder, carries a resistance element of annular configuration as well as a central contact. Mounted for rotation with the wormwheel is a wiper contact member having integral arms for frictional coupling to the wormwheel and having a third arm and resilient wiper for engaging the central contact and resistance element, respectively. Thus turning more specifically to the drawing a wormwheel 30 is employed having a recess 31 on one side which mates with the embossment 28 and having a stub shaft 32 projecting from the other side. Molded on the periphery of the wormwheel is a set of teeth 33 which mesh with a worm gear 34 on the adjusting screw. Adjacent the teeth 33 is a flange 35.

Seated on the internal shoulder 26 is a square wafer of substrate 40 having edges 41-44. Mounted upon the surface of the substrate is a resistance element 50 of interrupted annular shape terminating in conductive strips 51, 52. Lying between them is a third conductive strip 53 which makes connection with a central contact 55. The cermet resistance element may be formed using the procedure described in Ragan Patent 3,252,831 which issued on May 24, 1966. Briefly stated, a resistive mix is applied to the substrate by silk screening or the like and reduced in situ by appropriate firing steps, the mix being compounded to produce a desired terminal-to-terminal resistance when the reduction has been completed. The cermet element is distinguished by a high degree of stability, a low temperature coefficient, a wide range of available resistance, and long life in the face of repeated cycling of the wiper element.

Interposed between the wormwheel 30 and the cermet resistance element is a wiper member which is shown in some detail in FIGS. 6-8 of the drawings. This member is made of flat spring stock having integral arms 61-64 and a central opening which is dimensioned to register with the stub shaft 32 which projects from the wormwheel when the contact member is flatly seated on the wormwheel. The original shape of the arms, prior to forming, is shown in FIG. 6 by the dot-dash outline. For the purpose of providing frictional engagement between the contact member 60 and the wormwheel, the arms 61, 62 of the contact member are curled mutually inwardly so that the spacing d between them (see FIG. 7) is less than the diameter of the stub shaft 32. The smooth cylindrical surfaces inwardly presented by the arms 61, 62 permit the contact member 60 to simply press into place upon the stub shaft to develop an inward squeezing force, and adequate friction, without necessity for prying the arms apart for admission. Each curl extends at least through an angle of 270 suflicient to bring about a negator in which the force rises with displacement but at a decreasing rate. To establish an electrical connection with the center contact 55, the arm 63 is reversely bent and axially as shown in FIG. 8. When the arm 63 is pressed inwardly from the free position shown in the latter figure, the tip of the arm substantially coincides with the axis of the central contact.

For the purpose of making electrical connection with the cermet element 50, the integral arm 64 is reversely bent as shown in FIGS. 6 and 7 and the tip of the arms is suitably slotted and axially sprung to form a resilient wiper contact. A stop surface 66 is formed on the arm which carries the wiper. It will be understood that both the contact arm 63 and the wiper 64 are placed under axial compression when the wafer of substrate 40 is seated in its position within the housing.

In order to establish electrical contact with the conductive strip 51 at one end of the resistive element, the terminal lead 11 is flattened and formed at its tip with a hook 11a which is bent about the edge 43 of the substrate. The inner end of the flattened portion is formed into a second hook 11b which is bent about the opposite edge 44 of the substrate. This securely anchors the terminal lead to the substrate, either with or without a suitable adhesive. Similarly, the terminal lead 12 is formed with hook portions 12a, 12b, which are bent around the substrate edges 43, 44. Preferably the terminal leads are spaced as far apart as possible and extend along the lateral edges 41, 42 of the substrate so that the hooks occupy corner positions. For the purpose of making contact with the conductive strip 53, the terminal lead 13 is bent into a hook 13a and terminates in an upstanding tip 13b which serves as a stop to define the limits of rotary movement of the wiper contact member 60.

To accommodate the terminal leads 11, 12, when the .device is assembled together, holes 7173 (see FIG. 5)

are provided within the housing in the corner positions, the holes 71, 72 being active when a Type X mounting configuration is used. Arranged at the terminus of the holes 71, 72 are notches 81, 82. A third notch 83 centered between them serves to accommodate the central terminal lead 13. During assembly, as shown in FIG. 2, the terminal leads 11, 12 extend at right angles to the plane of the substrate. When the substrate is pressed into position the terminal leads enter the respective holes 71, 72. With the substrate firmly seated on the internal shoulder 26, the terminal leads 11, 12 are bent at an angle of 90 as shown in FIG. 3 into a position in which they extend parallel to the terminal lead 13. To insure intimate seating of the substrate upon the shoulder 26, thereby to prevent leakage of potting material subsequently applied, the internal shoulder 26 may be relieved in the corner positions as shown at 26a by an amount just suflicient to clear the hook portions of the terminal leads. Moreover, if desired, the inner walls of the housing may be formed with integral snap-in embossments as indicated at 85 which yield to enable the substrate to be snapped into its seated position.

In accordance with one of the aspects of the present invention, a sleevelike opening is provided for access of the adjusting screw or Worm gear, and the screw has a neck portion which includes a series of sharp-edged annular ridges having an interfering fit with the opening to insure that the worm gear remains in position once it has been pressed into place and to develop friction against turning movement so that, once set, the device remains stably in adjusted position. Thus, referring to FIG. 5, the neck portion adjacent the worm gear has a set of three annular ridges 91-93 having a diameter which is greater than the nominal diameter of the sleevelike opening formed in the housing. In assembling the worm gear, pressure is applied in the direction of the arrow shown in FIG, 5a preferably with high intensity sonic vibration. This causes the ridges 91, 93 to be advanced into the opening 95 with an interfering fit. The ridges are preferably of a ratchet-like cross section presenting an angled forward face to facilitate crowding into the opening. The sonic vibration raises the temperature of the thermoplastic material so that a portion is pressed ahead of the ridges in the form of ripples 96. When the crest of each ridge has passed the thermoplastic material crowds behind the ridges and, upon cooling, locks the ridges in place. Thus the adjusting screw, or worm gear, is not only locked in position but develops sufiicient friction at the surface of the ridges so that friction must be overcome in turning the head 16 in one direction or the other. This insures that the setting, once made, cannot be changed by the effects of shock or vibration. Moreover, the worm gear 34 is so dimensioned with respect to the teeth 33 of the wormwheel that the two elements are preloaded against one another when the wormwheel is inserted into its seated position upon the embossment 28. Thus lost motion or backlash within the units is reduced to zero. If desired, the annular ridges 91-93 may be made of successively larger diameter so that each acts upon fresh thermoplastic material upon advancement into the fully seated position.

In accordance with one of the aspects of the invention a second set of notches 101, 102 (see FIG. 2), communicating with the holes 72, 73, respectively, as well as a notch 103, may be provided in the side 22 to accommodate the terminal leads when the substrate 40 is rotated 90 to produce a mounting configuration Type W. The assembly procedure is the same, except that the terminal leads 11, 12 are inserted into the holes 72, 73 (see FIG. 5) and bent upwardly into position in the notches 101, 102, with the terminal lead 13 occupying the central notch 103. With the wafer of substrate fully seated against the internal shoulder 26, and with the substrate uppermost, the space above the substrate is then filled with epoxy resin as indicated at 105 which not only seals the substrate to the housing about its lateral edges but which runs down adjacent the terminal leads to seal the leads with respect to the housing. Alternatively, a dab of epoxy may be applied to each of the notches. The epoxy potting material moreover, being filled flush with the edges of the housing, provides electrical insulation and physical protection for the flattened portions of the terminal leads resulting in a neat and integrated, generally cubic unit which is convenient to handle and easy to mount.

While it is possible to provide two sets of notches for immediate interchangeability between the Type X and Type W mounts, it will be apparent that for longer runs of either type the molding die for the housing may be modified to provide for notches 81, 83 and holes 71, 72 for Type X or, alternatively, notches 101-103 and holes 72, 73 for the Type W. Indeed, the notches and holes may be eliminated entirely and terminal leads may be provided which are bent at 90 to the back face of the substrate Where a third type of mounting, Type P, is desired. It will be apparent, then, that the present trimmer potentiometer provides a high degree of versatility With only minor and inexpensive changes in construction.

The present trimmer potentiometer is ideally suited for use under conditions of severe shock and vibration. Because of the friction loading of the adjusting screw combined with the pre-load which exists between the gears, and the fact that the gears are statically and dynamically balanced, enables the holding of the set value accurately under all conditions which may be encountered. The reaction force of the bent-over tab 63 and wiper 64 of the contact member is adequate to keep the wormwheel fully seated in its working position.

By reason of the friction clutching, as the wiper member strikes the stop 13b, which defines the limits of rotation in each direction, controlled slippage takes place between the resilient arms 61, 62 on the wiper and the stub shaft 32 so that the unit is not endangered. The leads, it will be noted, are all firmly anchored so that no amount of stress applied to a lead is capable of damaging the internal mechanism.

As to the materials of construction, any durable and accurately moldable plastic may be employed for the housing and wormwheel, as, for example, glass-filled nylon. The material should be such as to form a tight bond with the potting material. The adjusting screw is preferably made of metal. Because of the use of a minimum number of parts, application and assembly are highly economical.

In the following claims the term resilient arms applied to the clutching elements is intended to include spring arms which engage the stub shaft and which may extend either outwardly or inwardly with respect to the blank of which the spring contact element is made.

We claim as our invention:

1. In a trimmer potentiometer having an overall dimension on the order of a small fraction of an inch, the combination comprising a housing open on one side and having an internal shoulder defining a receptacle having an embossment axially centered therein, a wormwheel having a central recess on one side for mating with the embossment and a stub shaft on the opposite side, a manually rotatable worm gear penetrating into one side of the housing for meshing with the wormwheel, a wafer of substrate seated on the internal shoulder for enclosing the housing and having an interrupted annular resistance element thereon as well as a central contact, leads penetrating the housing and connected to the ends of the resistance element and central contact respectively, and a wiper contact member formed of flat spring stock seated on the wormwheel and having a central opening registering with the stub shaft and four integral arms, the first and second arms being curled inwardly in opposition to one another for frictionally embracing the stub shaft, the third arm being reversely bent and axially sprung to engage the central contact on the substrate, and the fourth arm being bent and axially sprung to form a resilient wiper contact for engaging the resistance element.

2. In a trimmer potentiometer having an overall dimen-' sion on the order of a small fraction of an inch, the combination comprising a housing open on one side and having an internal shoulder defining a receptacle axially centered therein, a wormwheel rotatably seated in the receptacle and having a stub shaft projecting therefrom, a manually rotatable worm gear penetrating into one side of the housing for meshing with the wormwheel, a wafer of substrate seated on the internal shoulder for enclosing the housing, said Wafer having an interrupted annular resistance element thereon as well as a central contact, leads penetrating the housing and connected to the ends of the resistance element and central contact respectively, and a wiper contact member formed of flat spring stock seated on the wormwheel and having a central opening registering with the stub shaft, said contact member having resilient integral arms for frictionally embracing the stub shaft, said contact member further having a reversely bent arm axially sprung to engage the central contact on the substrate as well as a resilent wiper contact for making contact with the annular resistance element as the wormwheel is rotated.

3. The combination as claimed in claim 2 in which the ends of the resistance element and the central contact terminate in conductive strips which extend to one edge of the Wafer of substrate and in which the terminal leads lie flatly spaced side by side against the reverse side of the substrate with the tips thereof being bent and hooked over the edge of the wafer of substrate for respective electrical contact with the conductive strips.

4. The combination as claimed in claim 3 in which the internal shoulder is recessed to accommodate the hooks for flat seating of the substrate upon the shoulder thereby to prevent leakage of potting material into the space containing the wormwheel and the worm gear.

5. The combination as claimed in claim 3 in which the terminal leads connected to the ends of the resistance element extend along opposite edges of the wafer of substrate and are hooked about the four corners thereof and in which the four corners of the internal shoulder are recessed to provide accommodation for the hooked portions of the terminal leads thereby to permit flat seating of the substrate on the internal shoulder.

6. The combination as claimed in claim 3 in which the terminal leads connected to the ends of the resistance element extend at right angles to the Wafer of substrate for reception in registering axially-extending holes in the corners of the housing.

7. The combination as claimed in claim 6 in which the housing is provided with laterally extending notches at the terminus of said holes, the terminal leads being bent laterally in said notches to a position parallel to the wafer of substrate.

8. The combination as claimed in claim 3 in which the wafer is square for seating in the housing in alternate positions and in which the housing is provided with two sets of notches to accommodate exiting of the leads in the alternate 90 positions in which the leads extend (a) at right angles to the worm gear and (b) parallel to the worm gear for alternate styles of mounting.

9. The combination as claimed in claim 2 in which the wafer of substrate when seated on the internal shoulder is recessed below the edge of the housing and in which the remaining space is filled with inert potting material.

10. The combination as claimed in claim 9 in which the housing is molded of resilient plastic material in which snap-in embossments are provided on opposed walls spaced above the internal shoulder for detented holding of the Wafer of substrate against the shoulder during application of the potting material.

11. The combination as claimed in claim 3 in which the hooked portion of the terminal lead which is connected to the central contact is bent at right angles to provide a stop arranged in the path of movement of the resilient wiper contact to define the respective limits of wiper motion.

12. The combination as claimed in claim 2 in which the housing is formed of thermoplastic material having a sleevelike opening for the worm gear and in which the worm gear has a neck portion provided with a series of sharp-edgedannular ridges having an interfering fit with respect to the sleevelike opening in the housing for secure retention in the latter and to develop friction against turning movement.

13. The combination as claimed in claim 2 in which the housing is formed of thermoplastic material having a sleevelike opening for the worm gear and in which the worm gear has a neck portion provided with a series of spaced annular ridges having an interfering fit with respect to the sleevelike opening and having a ratchet tooth cross section to facilitate forceable entry in the face of the interference and positive retention plus development of friction against turning movement thereafter.

14. In a trimmer potentiometer having an overall dimension on the order of a small fraction of an inch, the combination comprising a housing defining a receptacle and open on one side, a wormwheel rotatably seated in the receptacle and having a stub shaft projecting therefrom, means for rotating the wormwheel, a wafer of substrate seated on the housing for enclosing the same, said wafer having an interrupted annular resistance element thereon as well as a central contact, leads penetrating the housing and connected to the ends of the resistance element and central contact respectively, and a wiper contact member formed of fiat spring stock seated on the wormwheel and having a central opening registering with the stub shaft, said contact member having a pair of integral outwardly extending arms curled inwardly toward the stub shaft in opposition to one another for frictionally embracing the stub shaft, said wiper contact member having a third arm for resiliently engaging the central contact on the substrate as Well as a resilient wiper for engaging the resistance element in a selected position as the worm gear is rotated, said central contact having an associated stop for limiting the rotation of the contact member beyond the end of the resistance element in either direction accompanied by slippage between the curled arms and the stub shaft.

15. The combination as claimed in claim 14 in which the resilient arms are curled over an angle substantially in excess of 270 sufiicient to exhibit negator action incident to uncurling thereof as the stub shaft is inserted into Working position.

References Cited UNITED STATES PATENTS 12/1958 Purdy 338202 XR 7/1963 Thumbusch 338162 THOMAS J. KOZMA, Primary Examiner 

